Why is the linespread factor as it is?
With the LaTeX standard classes (article
, book
, and report
) and no class options added, \normalsize
results in a font size (size of the largest glyphs in a font -- typically, braces) of 10pt
and a \baselineskip
(vertical skip between the base lines of two successive lines of type) of 12pt
. The ratio between font size and \baselineskip
is 1.2.
The linespread
command (which must be issued in the document preamble) may be used to change the \baselineskip
without changing the font size.
A possible definition of \onehalfspacing
and \doublespacing
is that the ratio between font size and \baselineskip
should be 1.5 resp. 2. Because the "basic" ratio for 10pt
is 1.2, a multiplier of 1.25 and (approximately) 1.667 has to be applied -- and this is basically what the setspace
package does. ("Basically" because it retains the ratio of 1.2 for footnotes and the like.)
The statement "For double-spacing you have to use 1.6 and for one-and-a-half spacing 1.3" amounts to either a rounding error, or being confused, or both.
Extending lockstep's answer to other document class options (like 11pt
and 12pt
) gives rise to the seemingly strange scaling factors used by the setspace
package:
10pt
(already discussed)setspace
defines the scaling factor to be1.25
for\onehalfspacing
and1.667
for\doublespacing
, since the "basic" ratio is1.2
(\normalfont
has a\baselineskip
of12pt
; seesize10.clo
)11pt
setspace
defines the scaling factor to be1.213
for\onehalfspacing
and1.618
for\doublespacing
, since the "basic" ratio is1.236
(\normalfont
has a\baselineskip
of13.6pt
; seesize11.clo
)12pt
setspace
defines the scaling factor to be1.241
for\onehalfspacing
and1.655
for\doublespacing
, since the "basic" ratio is1.208
(\normalfont
has a\baselineskip
of14.5pt
; seesize12.clo
)
As such, in the following hypothetical situation, a scaling factor of 2.4
(that is, \setstretch{2.4}
) would provide "triple spacing" in a document with normal font size of 16pt
and \baselineskip
of 20pt
.
All scaling factors are rounded to 3 digits after the decimal.
I recently tried to customize the line spacing in a ctex
document and this post served as one of my references. During my research, however, I found a mistake made by the setspace
package.
To further extend Werner’s answer, I want to point out that setspace
has set both factors for an 11pt
article wrong, unfortunately.
We find in size11.clo
that \normalsize
is defined by
\@setfontsize\normalsize\@xipt{13.6}%
The problem is that the “basic” ratio in 11pt
article is not 13.6/11
but rather 13.6/10.95
for historical reasons. This implies that for \onehalfspacing
, the factor should be around 1.2077205882
, not 1.213
; and for \doublespacing
, the factor should be around 1.6102941176
, not 1.618
.
I have no idea why article
inconsistently sets the body text leading/fontsize basic ratio to be 1.2
, 1.242
(not 1.236
) and 1.208
for 10pt
, 11pt
and 12pt
, respectively.